The Thyroid and Diving

The thyroid gland secretes thyroxin which is a hormone that helps control the rate at which we burn up carbohydrates (metabolic rate). Too much thyroxin causes hyperthyroidism (thyrotoxicosis) — too little causes hypothyroidism (myxedema).

What a diver needs to be concerned with is his/her body’s ability to function with the increased work load that hyperthyroidism places on your heart. Add to this increased workload the load of diving with heavy gear and your heart may not be able to handle it, in that the person with hyperthyroidism is prone to having attacks of paroxysmal atrial tachycardia or atrial fibrillation (episodes of rapid heart beating) that can leave the person unconscious or unable to function. This would be disastrous under water, even if you were just skin diving or snorkeling.

Atypical presentation, with cardiac or psychiatric symptoms, is common in men. Patients with thyroid ophthalmopathy frequently have difficulty in upward gaze. Corneal damage and optic neuropathy (inflammation of the optic nerve) can also occur.

Return to diving: Return to diving may be considered once the patient is euthyroid (normal thyroid level) on a stable dose of replacement medication if required. Patients with ophthalmopathy will need to be disqualified while undergoing treatment and may need to be disqualified permanently if treatment is unsuccessful. Mask damage to the eye is a strong possibility in these situations.

Data required for decision making: Endocrinology consultation, appropriate laboratory studies and ophthalmological consultation is also required if exophthalmos (protrusion of eyes) or other eye conditions are suspected. Annual confirmation of clinical and chemical euthyroid (normal) status is needed for continued diving.

Therapy: There are 3 main forms of therapy: medical treatment with methimazole or similar drugs; radioactive iodine; and surgery. Methimazole may cause side effects including vertigo and drowsiness, as well as agranulocytosis (bone marrow suppression). Diving is contraindicated in when their is bone marrow suppression due to the possibility of increased infections. Surgery is declining in popularity but may be the treatment of choice in females of childbearing age, because of the possibility of ovarian damage from the radioactivity. A small number of cases will require eye surgery.

Notes for consideration of the diver: Muscle pain, weakness and stiffness are the presenting symptoms in 25% of patients. Weakness and tremor can be mistaken for decompression accidents. Bulbar involvement can occur. With drug treatment, there is a 50% relapse rate, some cases relapsing early. With radioactive iodine, 10 to 15% of cases will be hypothyroid (low thyroid condition) within 2 years, and 50 to 60% will be hypothyroid within 20 years. A third of patients undergoing surgery will be hypothyroid within 10 years. Patients therefore have to review regularly for the rest of their lives. The complete remission rate (those that get well) after radioactive iodine is 86% with 60% developing myxedema (puffy swelling of low thyroid condition) after 10 years and a further 2-3% a year developing myxedema after that. Only 5% of patients with Graves’ disease (hyperthyroidism) will have ophthalmopathy (protruding eyes). More than 50% of cases of exophthalmos (protruding eyes) will get better spontaneously within 5 years with no other treatment than that of the underlying condition. Only 5% of patients will require eye surgery.

Nitrox Diving and the Thyroid

Nitrox is the mixture of increased amounts of oxygen in the breathing air of divers. Regular air is 20%; nitroz is mixed in 32%, 36% and higher. This allows for longer bottom times, reduced risks of decompression illness (less nitrogen) but also imposes a penalty of increased risk of oxygen toxicity. Certain drugs are sympathomimetic (mimicking the action of the sympathetic nervous system) and increase the metabolic rate, heart rate and rate at which O2 is utilized. Thyroid hormone, either Synthroid or thyroxin produced by the body or taken by mouth act in this manner. These drugs also have been found to increase the risk of oxygen seizures at shallower depths (pressures).

To my knowledge there have been no studies that have shown any increased risks to the person with hyperthyroidism, even untreated. However, the prudent person would certainly not dive if his thyroid functions were out of line — just as he/she should not play tennis or handball or some other physical exercise until they were “euthyroid” (normal function of the gland). One month of treatment is not usually long enough to become euthyroid — although I’ve had patients who have responded rapidly to medication. Your doctor should be the final arbiter in this matter.